1. Field of the Invention
The present invention relates to a disc file for the storage of retrievable, information data, (in a plurality of concentric data storage tracks) on a rotating disc, wherein the tracks are accessed by a radially mobile head.
2. The Prior Art
The recording capacity of a disc, rotating at fixed angular velocity, for data signals, is limited by the resolvability of recorded signals on that part of the disc which is at least distance from the centre of rotation.
Informational data signals are generally recorded on a disc which rotates at a fixed angular velocity in a plurality of concentric equally spaced circular tracks, each track having, as its centre, the centre of rotation of the disc. A radially mobile transducer, is positioned so as to interact with one track, or a small group of adjacent tracks, to perform a recording or retrieval operation.
Every transducer has associated with it a minimum resolvability. That is, the transducer will give a good response when asked to record or replay signals whose constituent parts are separated by large distances on the recording medium, but as this distance is reduced, the constituent parts of the signal approach, in their separation, the physical dimensions of the acting parts of the transducer. The transducer cannot then distinguish between the constituent parts of the signal and a smeared, degraded response is obtained.
Every recording medium, also, has associated with it, a limit on the closeness with which the constituent parts of the signal may be recorded before a degraded response will be obtained from the transducer. This limit is a function of the material of the recording medium and its style of construction. For example, the recording density on a magnetic disc is limited by the thickness of the magnetic coating on the disc, and by the size of surface defects in the coating.
The maximum number of polarity reversals per unit length, on the medium, is a set characteristic of a disc file. The least length of any data storage track is found at the innermost part of the recorded surface of the disc. The number of polarity reversals on the track of least length sets the number of polarity reversals per revolution of the disc. No matter what the radius of a particular track, the number of polarity reversals is the same as for the innermost track. The parts of the disc, of greatest distance from the centre of rotation, could be used to store more signal than they do if this limitation could be overcome.
A method, well known in the art, is that of clock zoning, where a disc file is provided with several clock generators for the control of data deposition and recovery at different frequencies. Each clock generator is associated with a particular group of radially adjacent tracks on the disc. In use, the clock generators are selected such that the reading and writing of data on the disc is carried on with a frequency which increases with increasing radius of the group of tracks to which an accessed data storage track belongs. This measure, while increasing the amount of data which may be stored on a disc, is generally regarded as cumbersome and costly, being generally reserved for head-per-track machines where performance is of more significance than cost.